1. Field of the Invention
The present invention relates to an optical component, and more particularly to a micro-structural optical component comprising a polymer coating film that is directly and monolithically formed on a substrate.
2. Description of the Prior Arts
As various portable electronic equipment and network devices have been put to practical use in recent years, development has further been demanded for a small-sized and light-weight optical device having high performance. As a main optical device, there are conventionally bulk-type optical disc devices or optical communication devices which are formed by combining various optical components such as a mirror or prism.
In these optical devices, an optical component with an mm order size manufactured by molding, cutting and polishing a glass or crystal material has been adhered and fixed in advance to the predetermined position of a substrate. However, it is difficult to downsize more the general optical component of this type from the viewpoint of its production process. Further, the process for position alignment in the assembling becomes complicated, thereby entailing a problem of poor mass production.
In order to solve the above problem, a method disclosed in Japanese Unexamined Patent Publication No. 2001-33604 has been proposed. In this method, an optical thick film comprising a polymer material and formed on a substrate is processed with a reactive ion etching or laser abrasion for directly forming a micro-structural optical component. This method has characteristics such that the processing is easy compared to the glass or crystal material as well as the position alignment of each component is unnecessary.
The above-mentioned method will be schematically explained with reference to FIG. 5. An optical device 19 is formed on a silicon substrate 15. The optical device 19 is provided with first and second optical path altering mirrors (12, 13), a polarizing separation prism 11 and photodiodes (14a, 14b) as a light receiving element. The polarizing separation prism 11 is arranged between the first and second optical path altering mirrors 12 and 13. The second optical path altering mirror 13 is arranged on the photodiodes (14a, 14b).
Incident light 16 incident from the normal direction of the substrate of the optical device 19 is refracted via the first optical path altering mirror 12, converted into light parallel to the substrate surface and directed to the polarizing separation prism 11. The polarizing separation prism 11 is composed of a material having birefringence such that the refractive index is different in polarized light (TE light) 17 having a plane of vibration parallel to the substrate and polarized light (TM light) 18 having a plane of vibration perpendicular to the substrate. When light is incident to this prism 11 with a large incident angle, the incident light 16 is divided into two polarized lights in the prism 11 since the refraction angle is different in each polarizing light. Each separated polarized light 17, 18 is refracted via the second optical path altering mirror 13, and then, directed to the respective photodiodes 14a, 14b, whereby electrical signals corresponding to the intensity of each polarized light are detected. Accordingly, the optical device 19 can be functioned as a polarized light detecting element in a pick-up device for a magneto-optic disc device.
Usable material for this polarizing separation prism 11 is, for example, some type of polyimide that shows optical anisotropy with an optical axis in the plane-normal direction only by forming a coating film (heated and sintered after coating). The polyimide coating film can be processed into a desired shape by the reactive ion etching or laser abrasion. The reactive ion etching is suitable for a processing of a large area, but the processing speed is slow such as 1 μm per minute. Compared to this, the laser abrasion utilizing photodecomposition has a characteristic of obtaining a processing speed such as 100 μm per second in the depth direction. The laser abrasion will be explained with reference to FIG. 6.
FIG. 6 shows a method for forming a tapered surface to a transparent plastic layer 21 with the laser abrasion. Excimer laser beam 23 transmitting through a laser abrasion mask 22 having a transmitting area of a rectangular shape is scaled down by a lens 24 and irradiated to a workpiece 25. When the workpiece 25 is moved in a direction shown by an arrow X with the excimer laser beam 23 irradiated thereto, the processing is performed from the right side of the workpiece 25 (the processed area is shown by a broken line). When the irradiation of the excimer laser beam 23 is stopped with the workpiece 25 moved, difference occurs in the processing amount between the right side and left side of the irradiated area since the total irradiating amount is different, to thereby form a tapered surface. A tapered surface having an optional inclined angle can be processed by adjusting the intensity of the excimer laser beam and the moving speed of the workpiece.
When the excimer laser beam is irradiated to the workpiece that is not moved, the rectangular shape of the laser abrasion mask is scaled down to be projected, so that a processed surface perpendicular to the substrate surface can be formed. The explanation of this method with reference to figures is omitted here.
On the other hand, the surface precision of the processed surface can be enhanced by newly laminating a film on the surface so as to embed the surface irregularities. In case where the surface to be processed is great, the usable method is, for example, the one in which liquid photo-curing resin is spin-coated and then cured by photo-irradiation. In case where the surface to be processed is small or where the three-dimensional shape of a μm order level is formed on the substrate, the target shape is distorted with the aforesaid embedding. Therefore, a film-formation with CVD is suitable. Particularly, the CVD formation of polyparaxylylene film does not require the high-temperature heating to the substrate (possible with room temperature), so that isotropic coating is possible for resin material having poor heat resistance (for comparison, CVD formation of a quartz requires that a portion to which a film is formed is kept at high temperature such as 400° C.).
However, some type of polymer material causes rib-like surface irregularities on the processed surface due to the aforesaid reactive ion etching process or laser abrasion process, thereby deteriorating the surface precision (smoothness) of the obtained optical component.
Further, especially in the laser abrasion process, the processed area is modified by the irradiation of the excimer laser due to a thermal reaction, thereby entailing a problem of not obtaining a smooth processed surface.